<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>

	<body>
		<input id="num1" />+<input id="num2" />
		<input type="button" value="=" onclick="jisuan()" />
		<input id="num3" />
		<script>
			var jisuan = function() {
				num3.value = parseFloat(num1.value) + parseFloat(num2.value);
			}
		</script>
	</body>

</html>